The present invention relates to an optical multiplexing and de-multiplexing element usable for multiplexing and de-multiplexing of light components having different wavelengths in wavelength division multiplex (WDM) communication and the like, and an arrayed-waveguide grating-type optical wavelength filter provided with this optical multiplexing and de-multiplexing element.
Recently, as an optical subscriber access system, a passive optical network (PON) communication system has become mainstream in which one optical line terminal (OLT) and a plurality of subscriber-side optical network units (ONUs) are connected via optical fibers and a star coupler, and one OLT is shared by the plurality of the ONUs. In this communication system, an optical signal wavelength used in down-link communication and an optical signal wavelength used in up-link communication are made different so as to cause the down-link communication from the OLT toward the ONU and the up-link communication from the ONU toward the OLT not to interfere with each other.
In the optical subscriber access system, further there has been studied a wavelength division multiplexed-PON (WDM-PON) which improves the multiplicity of wavelengths to be used for communication. The WDM-PON needs an optical element to perform the multiplexing and de-multiplexing of optical beams having a plurality of wavelengths for the OLT and the ONU.
An example of such an optical element is an arrayed waveguide grating (AWG). In the AWG, an input waveguide, an input-side slab waveguide, an arrayed waveguide including a plurality of channel waveguides having different optical path lengths, an output-side slab waveguide and an output waveguide are formed on the same substrate as a planar lightwave circuit (PLC). However, in a quartz light waveguide having a small refraction difference between a core and a clad, it is difficult to reduce the curvature radius of a curved light waveguide and the AWG cannot be made small.
Accordingly, an example of configuring the AWG is reported for a silicon fine wire waveguide using a core made of silicon (Si) and a clad made of silicon oxide (SiO2) having a large refraction difference relative to silicon (refer to non-patent literature 1 (Wim Bogaerts, et al., “Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 16, No. 1, pp. 33 to 44 (2010)), and non-patent literature 2 (Daoxin Dai et al., “Ultrasmall Overlapped Arrayed-Waveguide Grating Based on Si Nanowire Waveguides for Dense Wavelength Division Demultiplexing”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 12, No. 6, pp. 1301 to 1305 (2006)), for example). In the silicon fine wire waveguide, the refractive index of a core is extremely larger than the refractive index of a clad, and therefore the curved light waveguide having strong confinement of light and having a sufficiently small curvature radius can be formed. Further, since the silicon fine wire waveguide can be manufactured by the use of the process technique for a silicon electronic device, it is possible to realize a cross-sectional structure having an extremely fine submicron size. Accordingly, it is possible to make the AWG small by using the silicon fine wire waveguide.
However, in the AWG using the silicon fine wire waveguide, there is known that a non-negligible radiation loss is caused at a connection part between an input-side or output-side slab waveguide and a channel waveguide configuring the AWG. As a method of reducing this radiation loss, a rib waveguide structure is tried to be used for the waveguide configuring the AWG. However, when the rib waveguide structure is employed, a non-negligible loss is caused in this part unless the curvature radius of the curved waveguide part is increased. Accordingly, there has been a trial of reducing the radiation loss by using a special rib waveguide (refer to non-patent literature 3 (Jaegyu Park, et al., “Improved performance of a silicon arrayed waveguide grating by reduction of higher order mode generation near the boundary of a star coupler”, Proceedings of SPIE vol. 9367, pp. 936705-1 to 6 (2015)).
Further, patent literature 1 (U.S. Pat. No. 6,442,308 B1) discloses that, for the radiation loss caused at the connection part between the input-side or output-side slab waveguide and the channel waveguide configuring the AWG, a waveguide mode coupler is utilized for equalizing a radiation loss caused in each of a plurality of waveguides configuring the channel waveguide.